Condensation heat-transfer device

ABSTRACT

The heat transfer surfaces of a condensation heat exchanger are provided with a coating according to the invention which comprises a sequence of layers which includes at least one hard layer which comprises amorphous carbon or a plasma polymer and at least one soft layer which comprises amorphous carbon or a plasma polymer. The hard and soft layers are applied alternately, the first layer on the heat transfer surface being a hard layer and the last layer of the coating being a soft layer. The last, soft layer is distinguished in particular by hydrophobic properties. The layer sequence ensures condensation of drops and, at the same time, protects against drop impingement erosion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a condensation heat exchanger for condensationof nonmetallic vapors, and in particular to a coating of the heattransfer surfaces of the condensation heat exchanger. The coating isused to extend the service life of the cooling tubes and to improve theheat transfer at the heat transfer surfaces.

2. Discussion of Background

In condensation heat exchangers, the lifespan of the heat transfersurfaces plays an important role, since damage to the heat transfersurfaces causes the entire installation in which the condensation heatexchanger is installed to fail. The state of the heat transfer surfacesof condensation heat exchangers is adversely affected, inter alia, bydrop impingement erosion and corrosion. Damage caused by dropimpingement erosion occurs in particular at those heat transfer surfaceswhich are exposed to a high-speed flow of steam. There, drops which arepresent in the steam which is to be condensed impinge on the heattransfer surfaces, energy being transferred to the surface by the impactor by shear forces. Erosion occurs if, in the event of very frequentdrop impingement, the energy transferred is sufficient to plasticallydeform the surface material, leads to creep in the case of ductilematerials or leads to intercrystalline fatigue failure in the case ofhard materials.

With steam condensers in steam power plants, it has been observed thatrelatively large drops with diameters in the region of 100 μm andvelocities of 250 m/s cause drop impingement erosion. In particular thecooling tubes at the periphery of a tube bundle are effected, while thetubes in the interior of a tube bundle remain protected from direct dropimpingement erosion.

The occurrence of drop impingement erosion is highly dependent on thematerials properties, such as hardness, ductility, elasticity,microstructure and roughness, materials made from titanium and titaniumalloys being distinguished by a certain resistance to erosion, althoughthis resistance is insufficient and predominantly due to their highhardness. In the case of steam condensers used in steam power plants,drop impingement erosion of this type is inhibited by a suitableselection of material for the cooling tubes, such as for example forstainless steel, titanium or chromium.

Furthermore, drop impingement erosion presents a problem in particularat low condenser pressures and therefore relatively high vaporvelocities, as for example in steam condenses in steam power plantswhich are operating at part loads. When steam condenses on heat transfersurfaces, according to the prior art a film of condensate which spreadsover the entire surface is formed. This film of condensate increases theoverall thermal resistance between steam and cooling liquid flowing inthe tubes, with the result that the heat transfer capacity is reduced.For this reason, there have long been efforts made to provide heattransfer surfaces with a coating which, by dint of hydrophobicproperties, prevent the formation of a film of condensate, so that dropcondensation occurs at the surface. The formation of drops allows thecondensate to run off more quickly than if a film is formed. As aresult, the surface of the heat exchanger is cleared, so that vapor cancondense again at the surface without being impeded by a film ofcondensate. The overall heat resistance therefore remains relativelylow. By way of example, layers of Teflon or enamel have been tried forthis purpose, but without great success, since these layers had littleresistance to erosion and corrosion.

In terms of the coating, it is important to solve the problem of theresistance to erosion and corrosion and also that of the bonding of thecoating to the heat transfer surfaces. These problems need to be solvedin particular in view of the desired long operating time of thecondensation heat exchanger, such as for example in the cooling tubes ofa steam condenser, which have to be able to operate for a period ofseveral years.

An example of a coating is disclosed by WO 96/41901 and EP 0 625 588.These documents describe a metallic heat transfer surface with what isdescribed as a hard-material layer comprising plasma-modified amorphoushydrocarbon layers, also known as diamond-like carbon. Amorphous carbonis known for its elastic, extremely hard and chemically stableproperties. The wetting properties of the hard-material layer ofamorphous carbon are altered by the incorporation of elements such asfluorine and silicon, in such a manner that the layer acquireshydrophobic properties. For bonding to the substrate, an interlayer isapplied between the substrate and the hard-material layer, thetransition from the interlayer to the hard-material layer being producedby means of a gradient layer. However, the hard-material layer isultimately only resistant to erosion on account of its inherenthardness.

DE 34 37 898 has described a coating for the surfaces of a heatexchanger, in particular for the surfaces of condenser cooling tubes,comprising a triazine-dithiol derivative. This layer material effectsdrop condensation and therefore improves the heat transfer. Furthermore,the coating is distinguished by good bonding to the cooling tubes.

DE 196 44 692 describes a coating comprising amorphous carbon whichbrings about drop condensation on the cooling tubes of steam condensers.The surface of a cooling tube is roughened prior to the application ofthe amorphous carbon, with the result that the effective interfacebetween the cooling tubes surface and the coating is increased. As aresult, the heat resistance between coating and base material isreduced. After the coating, the surface is smoothed, so that coated anduncoated regions are formed next to one another.

SUMMARY OF THE INVENTION

Accordingly, one object of the invention is to provide a novel coatingfor the heat transfer surfaces of a condensation heat exchanger for thecondensation of nonmetallic vapors, the resistance of which with respectto drop impingement erosion and corrosion is increased compared to theprior art and at which, at the same time, improved heat transfer iseffected as a result of drop condensation being brought about.

This object is achieved by a condensation heat exchanger in accordancewith claim 1. The heat transfer surfaces of a condensation heatexchanger have a coating which contains amorphous carbon, also known asdiamond-like carbons. According to the invention, the coating comprisesa layer sequence including at least one hard layer made from amorphouscarbon and at least one soft layer made from amorphous carbon, the hardand soft layers being applied alternately and the bottom or first layeron the heat transfer surface being a hard layer and the top or lastlayer of the layer sequence being a soft layer. The last, soft layer ofthe layer sequence in particular has hydrophobic or water-repellentproperties.

The coating according to the invention therefore, on account of its lastor outermost layer, makes the entire layer system hydrophobic. Thisproperty is based on the low surface energy of amorphous carbon when itis relatively soft.

In the text which follows, the term amorphous carbon is to be understoodas meaning hydrogen-containing carbon layers with a hydrogen content of10 to 50 atomic % and with a ratio of sp³ to sp² bonds of between 0.1and 0.9. In general, it is possible to use all amorphous or dense carbonlayers produced by means of carbon or hydrocarbon precursors as well asplasma polymer layers, polymer-like or dense carbon and hydrocarbonlayers, provided that they have the hydrophobic properties and also themechanical or chemical properties of amorphous carbon mentioned belowfor the production of layer sequences.

The wettability of the surface made from amorphous carbon can be alteredby varying its hardness. The higher the hardness, the lower thewettability becomes. A very hard layer with a hardness of, for example,more than 3000 Vickers would be less suitable as an outermost,hydrophobic layer than a layer of lower hardness.

The formation of extended films of condensate on the soft, hydrophobicsurface is prevented since the condensate instead forms drops which,once they have reached a certain size, run off the surface of the tube.In this case, on the one hand a larger part of the area of the heattransfer surface remains free of condensate, and on the other hand theresidence time of the condensate on a given heat transfer surface isalso greatly reduced. This increases the heat transfer at the surfacesand ultimately improves the performance of the condensation heatexchanger.

The layer sequence according to the invention, comprising in each case ahard layer followed by a soft layer, in particular effects an increasedresistance to drop impingement erosion. The impulse of impinging dropsis absorbed by the soft and hard layers by the compression waves whichoriginate in the surface material from the impingement of the dropsbeing attenuated by interference by the pairs of hard and soft layers.This attenuation of compression waves is similar to the attenuation ofoptical waves brought about by layer pairs of thin layers with a highand low refractive index, respectively.

The attenuation of compression waves is increased by a layer sequencecomprising a plurality of layer pairs of hard and soft layers. Anoptimum number of layers is dependent on the angle of inclination of thedirection of impingement of the drops on the surface. If they impingeobliquely, a smaller number of layers is required to attenuate thecompression waves.

The overall heat resistance of the coated heat transfer surfaceincreases as the number of layers and the layer thickness rise.Therefore, the number of layers is to be optimized on the basis of theabsorption of the compression waves which originate from the impingementof drops and also on the basis of the total heat resistance of the heattransfer surfaces.

The combination of one or more layer pairs of hard and soft layersproduces a greatly improved resistance to erosion compared to coatingscomprising amorphous carbon with only one layer of relatively highhardness. At the same time, on account of its outermost, soft layer, thecoating according to the invention has the ability to form dropcondensation. As a result, increased resistance to drop impact erosionand, at the same time, a high heat transfer on account of the increasedproportion of the area of the heat transfer surface which is free ofcondensate are ensured, so that both a lengthened service life of theheat transfer surfaces and an increased capacity on the part of thecondensation heat exchanger are achieved.

The coating according to the invention is eminently suitable for thecooling tubes of condensation heat exchangers. The cooling tubes atwhich vapor of any described material is precipitated are arrangedvertically or horizontally in tube bundles. In the case of a steamcondenser, such as for example in a steam power plant, in particular thecooling tubes at the periphery of a tube bundle are more exposed to thedrops which flow in at a high velocity than cooling tubes in theinterior of a bundle. The two-layer or multi-layer coating is thereforeparticularly suitable for those cooling tubes which lie at theperiphery. The cooling tubes in the interior of the bundle can beprovided with the same coating or merely with a single, soft hydrophobiclayer of amorphous carbon. This effects drop condensation with theassociated increase in heat transfer. There is less need to protectagainst drop impingement erosion there.

As has been mentioned, the drop condensation reduces the residence timeof the condensate on the cooling tubes of the steam condenser. Thisresults in a reduction in the vapor-side pressure drop, the pressuredrop being dependent on the size of the tube bundle and the volume ofthe condensate and on the plate width. The reduction in the vapor-sidepressure drop produces an improvement in the overall heat transfercoefficient. Compared to condensers with uncoated cooling tubes, it ispossible to increase the heat transfer coefficient by at least 25%, withthe condensation heat exchanger being able to condense up to 20% moresteam.

Furthermore, the coating is suitable for protecting against erosion andcorrosion in heat exchangers, such as for example against ammoniaerosion in steam condensers with heat transfer surfaces made in copperalloys. A further application is to protect against SO₃ or NO₂ corrosionin condensers used in apparatus for recuperation of heat from stackoff-gases. In this application, the interfacial energy must be very lowcompared to the surface tension of the condensate. Since the surfacetension of sulphuric acid is lower than that of water, therefore, theinterfacial energy of the outermost layer generally has to be lower thanin steam condensers. In this case, the hardness of the outermost layershould be between 600 and 1500 Vickers.

Furthermore, the coating according to the invention can be used infurther condensation heat exchangers, such as for example inrefrigeration machines and indeed any heat exchangers in whichcondensation takes place and drop impingement erosion has to beprevented.

The coating according to the invention can be produced using various,generally known production processes, such as for example deposition bymeans of glow discharge in a plasma comprising hydrocarbon-containingprecursors, ion beam coating and sputtering of carbon inhydrogen-containing working gas. In these processes, the substrate isexposed to a current of ions of several 100 eV. During the glowdischarge, the substrate is arranged in a reactor chamber in contactwith a cathode which is capacitively connected to a 13.56 MHz RFgenerator. The grounded walls of the plasma chamber form a largercounter electrode. In this arrangement, it is possible to use anyhydrocarbon vapor or an hydrocarbon gas as first working gas for thecoating. To achieve particular layer properties, for example differentsurface energies, hardnesses, optical properties, etc., various gasesare added to the first working gas. By way of example, high or lowsurface energies are achieved by adding nitrogen, fluorine- orsilicon-containing gases. The addition of nitrogen additionally leads toan increase in the hardness of the layer which results. Furthermore,changing the bias voltage across the electrodes between 100 and 1000 Vmakes it possible to control the resulting hardness of the layer, a highbias voltage leading to a hard, amorphous carbon layer and a low voltageleading to a soft, amorphous carbon layer.

In an exemplary embodiment, the hardness of a hard layer of a layer pairis between 1500 and 3000 Vickers, while the hardness of a soft layer ofa layer pair is between 800 and 1500 Vickers. The thicknesses of theindividual layers are between 0.1 and 2 μm, preferably between 0.2 and0.8 μm, if a plurality of layers are applied in succession in the layersequence. The total layer thickness is in this case in the range from 2to 10 μm, preferably between 2 and 6 μm. The thicknesses of the harderand softer layers are preferably inversely proportional to theirhardnesses.

The coating according to the invention includes at least one layer paircomprising a hard layer and a soft layer. In this case, a larger numberof layer pairs can be achieved, such as for example two layer pairs ineach case comprising one hard layer and one soft layer, provided thatthe layer sequence begins with a hard layer and ends with a soft layerwith hydrophobic properties. The greater the number of layers, thebetter the attenuation of the impingement energy works, but also thehigher the heat resistance becomes, since the hard and soft layers havedifferent thermal conductivities.

The coating according to the invention bonds well to most types ofsubstrates, in particular with materials which form carbides, such asfor example titanium, iron and silicon as well as aluminum, but not toprecious metals, copper or copper-nickel alloys. In this case, it is notnecessary to roughen the substrate surface to improve the bonding. Ifthe coating is applied to a smooth substrate surface, the result is alayer assembly which is even more resistant to drop impingement erosion,since this reduces the absorption of the impact energy by the basematerial. Therefore, the coating according to the invention can beapplied to various substrate materials which are used for the heattransfer surfaces, such as for example titanium, stainless steel,chromium steels, aluminium and all carbide-forming elements.

1. A condensation heat exchanger comprising: heat transfer surfaces forcondensation of nonmetallic vapors, the heat transfer surfaces having acoating which contains amorphous carbon, and the surface of the coatinghaving hydrophobic properties; wherein the coating, in order toattenuate compression waves which originate from the surface of thecoating as a result of the impingement of drops, comprises two or morelayer pairs, each layer pair including a hard layer comprising amorphouscarbon or a plasma polymer and a soft layer comprising amorphous carbonor a plasma polymer, and the hard and soft layers being appliedalternately and the last layer being a soft layer.
 2. The condensationheat exchanger as claimed in claim 1, wherein the thickness of each ofthe hard and soft layers is inversely proportional to their hardness. 3.The condensation heat exchanger as claimed in claim 1, wherein the hardlayers each have a hardness in the range from 1500 to 3500 Vickers andthe soft layers have a hardness in the range from 600 to 1500 Vickers.4. The condensation heat exchanger as claimed in claim 1, wherein thethickness of each of the hard and soft layers of the coating is between0.1 and 2 micrometers.
 5. The condensation heat exchanger as claimed inclaim 1, wherein the coating includes a plurality of layer pairs in eachcase comprising a hard layer and a soft layer, and the total thicknessof the coating is between 2 and 10 micrometers.
 6. The condensation heatexchanger as claimed in claim 1, wherein the heat transfer surfacescomprise titanium, stainless steel, chromium steel, aluminum, copperalloys or carbide-forming elements.
 7. The condensation heat exchangeras claimed in claim 1, wherein the coating inhibits ammonia erosion orcorrosion.
 8. The condensation heat exchanger as claimed in claim 1,further comprising: tube bundles comprising a plurality of cooling tubeswhich are arranged vertically or horizontally and on which vapor of anydesired substance can precipitate; outer cooling tubes at the peripheryof the tube bundles having the coating comprising at least one hardlayer and at least one soft layer; and inner cooling tubes of thebundles having the same coating or a coating comprising only a soft,hydrophobic layer comprising amorphous carbon.